Low-temperature impact-resistant polyamide-based stretch-oriented multilayer film

ABSTRACT

A stretch-oriented multilayer film suited for use as a freeze packaging material, a deep drawing packaging material, a vertical pillow packaging material, etc., is provided as a stretch-oriented multilayer film, comprising at least three layers including a surface layer (a) comprising a thermoplastic resin, an intermediate layer (b) comprising a polyamide resin and a surface layer (c) comprising a sealable resin, said multilayer film exhibiting an impact energy of at least 1.5 Joule at a conversion thickness of 50 μm at −10° C. The multilayer film is produced through an inflation process using water having a large capacity as a cooling and a heating medium and including a combination of a high degree of stretching and a high degree of relaxation heat treatment not exercised heretofore.

TECHNICAL FIELD

The present invention relates to a stretch-oriented (i.e., stretched andoriented) multilayer film including a polyamide resin layer as aprincipal resin layer and having an excellent low-temperature impactresistance.

BACKGROUND ART

Hitherto, it has been widely practiced to use a stretch-oriented filmfor packaging processed livestock meat, fresh fish meat, raw meat, soup,etc. Particularly, for packaging of frozen food and vacuum packaginglivestock meat with bones and fish meat, such a stretch-orientedpackaging film is required to have excellent anti-pinhole property andmechanical strength, and a multilayer film including a polyamide resinlayer has been generally used therefor.

Particularly, in the case of requiring a relatively thin multilayerfilm, a multilayer film including a stretched polyamide resin layer hasbeen suitably used.

For production of a multilayer film including a stretched polyamideresin layer, there has been generally practiced a process comprising drylamination or extrusion lamination of an unstretched polyolefin resinlayer onto a stretched polyamide resin layer. However, in the case ofnecessitating excellent anti-pinhole property and mechanical strength,such a lamination film is caused to have a complicated layer structure,which requires an increased number of production steps and an increasedfilm thickness for ensuring the strength, thus leading to an increasedproduction cost.

For the purpose of reducing the production steps, the multilayerstretching of a co-extruded multilayer film has been commercialized, butthis is accompanied with a problem such that the produced film is liableto have an excessively large shrinkability, which makes difficult thesecondary processing, such as bag-making, printing or packaging by meansof an automatic packaging machine. Particularly, in the case of packinga soup in a hot state into a pouch of such a multilayer film or byautomatic packaging with such a multilayer film, the packing becomesdifficult due to the film shrinkage.

It has been known that a multilayer film including a stretched polyamideresin layer has an excellent low-temperature strength, but it has beenalso believed inevitable that the strength is remarkably lowered at atemperature as low as −10° C. for packaging a frozen product.

DISCLOSURE OF INVENTION

A principal object of the present invention is to provide apolyamide-based stretch-oriented multilayer film having an improvedlow-temperature impact resistance.

Another object of the present invention is to provide a polyamide-basedstretch-oriented multilayer film excellent in mechanical properties,such as piercing strength and anti-pinhole property.

A further object of the present invention is to provide apolyamide-based stretch-oriented multilayer film which is free fromexcessive shrinkability and is excellent in boiling resistance,processability as by deep drawing or adaptability to packaging machines.

According to our study, it has become possible to obtain apolyamide-based stretch-oriented multilayer film having a remarkablyimproved low-temperature impact resistance while retaining ordinaryphysical properties such as a tensile strength by duly noting theposition of the polyamide resin layer and effecting appropriatestretching process and post-heat treatment. It has been also discoveredthat the multilayer film is also provided with remarkable improvement inthe above-mentioned objective physical properties and processingcharacteristics, which are presumably attributable to a molecularorientation state of the film having provided the improvement inlow-temperature impact resistance.

According to the present invention based on the above knowledge, thereis provided a stretch-oriented multilayer film, comprising at leastthree layers including a surface layer (a) comprising a thermoplasticresin, an intermediate layer (b) comprising a polyamide resin and asurface layer (c) comprising a sealable resin, said multilayer filmexhibiting an impact energy of at least 1.5 Joule at a conversionthickness of 50 μm at −10° C.

The present invention further provides a preferred process for producingthe above-mentioned stretch-oriented multilayer film. Thus, according tothe present invention, there is also provided a process for producing astretch-oriented multilayer film, comprising the steps of:

co-extruding at least three species of melted thermoplastic resins toform a tubular product comprising at least three layers including anouter surface layer (a) comprising a thermoplastic resin other thanpolyamide resin, an intermediate layer (b) comprising a polyamide resinand an inner surface layer (c) comprising a sealable resin,

cooling with water the tubular product to a temperature below a lowestone of the melting points of the thermoplastic resin, the polyamideresin and the sealable resin constituting the layers (a), (b) and (c),

re-heating the tubular product to a temperature which is at most thelowest one of the melting points of the thermoplastic resin, thepolyamide resin and the sealable resin constituting the layers (a), (b)and (c),

vertically pulling the tubular product while introducing a fluid intothe tubular product to stretch the tubular product in the verticaldirection and the circumferential direction, thereby providing abiaxially stretched tubular film,

folding the tubular film,

again introducing a fluid into the folded tubular film to form a tubularfilm,

heat-treating the tubular film from its outer surface layer (a) withsteam or warm water until a relaxation ratio reaches at least 20% in atleast one of the vertical direction and the circumferential direction,and

cooling the heat-treated tubular film to provide a stretch-orientedmultilayer film exhibiting an impact energy of at least 1.5 Joule at aconversion thickness of 50 μm at −10° C.

Some history and details as to how we have arrived at the presentinvention as a result of study for achieving the above object, will nowbe briefly discussed.

As mentioned above, it has been well known that a multilayer filminducing a polyamide resin layer as a principal layer provides apackaging material excellent in various properties. Our research grouphas also disclosed a stretch-oriented multilayer film having a basiclaminate structure of polyester resin/polyamide resin/sealable resin asa heat-shrinkable film with excellent physical properties (JP-A 4-99621,U.S. Pat. No. 5,336,549), and has further disclosed that a multilayerfilm having such a laminate structure can achieve an effective reductionin heat-shrinkage stress which can be obstacle to automatic packaging,while retaining a preferred degree of heat-shrinkability (JP-A11-300914, WO 99/55528). Particularly, the latter WO '528 publicationdiscloses a process including steps of subjecting a biaxially stretchedtubular film of the above-mentioned laminate structure after stretchingat a ratio of 2.5–4.0 times both in a vertical direction and in acircumferential direction to a heat treatment with steam or warm waterat 60–98° C. and then cooling the heat-treated film, to proved abiaxially stretched film exhibiting a heat-shrinkage stress at 50° C. ofat most 3 MPa both in longitudinal direction and in transversedirection, and a hot water shrinkability at 90° C. of at least 20%. Theinvention of the WO '528 publication aims at production of aheat-shrinkable film, and in its Examples, the relaxation ratio in thepost-heat treatment after the biaxial stretching remained at a level of15% at the most. In contrast thereto, we have tried a heat treatmentwith steam or warm water causing a larger relaxation ratio of 20% orhigher after a biaxial stretching at ratios identical to or even higherthan those adopted in the WO '528 publication. As a result thereof, ithas been unexpectedly found possible to obtain a remarkably improvedlow-temperature impact resistance represented by an impact energy at−10° C. while retaining ordinary film strength as represented by atensile strength.

As an ordinary practice, a highly stretched film is not daringlysubjected to heat treatment for a high degree of relaxation. A firstreason thereof is that a heat treatment for a high degree of relaxationhas not been believed desirable since such a relaxation heat treatmentfunctions to deliberately lower a rigidity and a strength of a film, theincrease of which is a principal purpose of a high degree of stretching.More specifically, a relaxation heat treatment of a highly stretched,even if performed, should be suppressed to such a level as to moderate adifficulty accompanying a high degree of stretching, i.e., anexcessively large heat shrinkage stress, and a higher degree ofrelaxation heat treatment, if performed, has been believed to onlyresult in a decrease of the effect of the high degree of stretching andnot provide an additional effect. As another reason, one advantageattained by a high degree of stretching is the possibility of obtaininga film of a large width by using a film forming apparatus of arelatively small scale. However, a high degree of relaxation heattreatment functions to completely negate the effect, and requires alarge scale apparatus for producing a film of a large width. Further, ahigh degree of relaxation heat treatment lowers the productivity of filmon an area-basis and can result in a remarkably lower yield due tooccurrence of film products failing to satisfy a regulation of width.

Unexpectedly, however, it has been found possible to obtain a remarkableincrease in low-temperature impact resistance by the process of thepresent invention represented by the above-mentioned combination of highdegree of stretching and high degree of relaxation heat treatment. Thereason thereof has not been fully clarified as yet but may be presumedas follows. The film of the present invention at a stage after a highdegree of stretching (that is inevitably required for allowing asubsequent high degree of relaxation heat treatment since a degree ofrelaxation exceeding a hot water shrinkability of a film after thestretching treatment is impossible) is composed of film-constitutingmolecules including a crystalline portion and an amorphous portion whichhave been both molecular-oriented to provide increased tensile strength,etc. The film in this state is liable to show a reduced elongation atbreakage, but the orientation of the amorphous portion is sufficientlyrelaxed owing to a subsequent high degree of relaxation heat treatmentwhile retaining the orientation of the crystalline portion, whereby aremarkable increase in elongation at breakage is understood to resultwhile retaining absolute strengths such as a tensile strength. Further,the increased elongation at breakage due to the relaxation of theamorphous portion is understood to be associated with improved otherfilm properties also aimed at by the present invention, i.e.,improvements in piercing strength, anti-pinhole property, deep-drawingprocessability requiring easiness of elongation, vertical pillowpackaging (or vertical type forming, filing and closing packaging)characteristic requiring a thin, pliable but tough film,lid-adaptability and boiling resistance requiring low shrinkability,etc.

As described above, in order to obtain a stretch-oriented multilayerfilm having an improved low-temperature impact resistance according tothe present invention, a combination of a high degree of stretching anda high degree of relaxation heat treatment is essential. For realizingthe combination, it is essential to include a principal resin layercomprising a polyamide resin which is adapted to a high degree ofstretching and acquires remarkably improved mechanical strength thereby.In the process of the present invention, the polyamide resin layer isfurther used as an intermediate layer to realize the high degree ofstretching—high degree of relaxation heat treatment. More specifically,in order to allow the high degree of stretching—high degree ofrelaxation heat treatment, the resinous tubular product (parison) afterthe heat melting extrusion is quenched with water as a cooling mediumexhibiting good heat efficiency, whereby a stretching stress iseffectively applied to resin molecules in the subsequent biaxialstretching step. Further, also in the subsequent post-heat treatment, ahigh degree of relaxation heat treatment is performed effectively byusing steam or warm water as a heating medium having a large heatcapacity. However, the polyamide resin is moisture-absorptive, so thatif the polyamide resin is exposed to the surface layer, the resinoustubular product after the melt extrusion absorbs water at the time ofwater quenching to lower the effect of the high degree of stretchingtreatment. Accordingly, in the process of the present invention, apolyamide resin layer excellent in stretchability and mechanicalproperties after the stretching is used as a principal intermediatelayer, and water showing excellent thermal efficiency as heating andcooling media to realize the high degree of stretching—high degree ofrelaxation heat treatment, thereby succeeding in production of astretch-oriented multilayer film having a remarkably increasedlow-temperature impact resistance.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 the sole FIGURE in the drawing, is a schematic illustration of anapparatus system suitable for practicing an embodiment of the processfor producing a stretch-oriented multilayer film according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The stretch-oriented multilayer film according to the present inventioncomprises at least three layers including a surface layer (a) comprisinga thermoplastic resin, an intermediate layer (b) comprising a polyamideresin and a surface layer (c) comprising a sealable resin.

The thermoplastic resin constituting the surface layer (a) is requiredto provide a surface layer (a) which, in the state of lamination withthe intermediate layer (b) comprising a polyamide resin, is required toexhibit an appropriate degree of stretchability and obstruct moisturepenetration to the intermediate layer. The thermoplastic resin maypreferably have a lower moisture-absorptivity than the polyamide resin.Preferred examples of the thermoplastic resin may include: polyolefinresins which have been conventionally used for polyamide resin-basedlaminate films, inclusive of polyethylenes, such as LLDPE (linearlow-density polyethylene), VLDPE (linear very low-density polyethylene)and LDPE (low-density polyethylene) (polyethylenes herein includingthose polymerized with single-site catalysts (or metallocene catalysts)in addition to those polymerized by conventional catalysts(Ziegler-Natta catalysts)); polypropylene, propylene-ethylene copolymer,propylene-ethylene-butene-1 copolymer, ethylene-vinyl acetate copolymer,ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer,and ethylene-ethyl acrylate copolymer, wherein comonomers other thanolefin occupy a relatively minor proportion (below 50 wt. %); and alsopolyester resin, etc. Among these, a polyester resin is excellent insurface properties, such as transparency, surface hardness, printabilityand heat resistance, and is a particularly preferable material for thesurface layer (a).

The polyester resin (PET) preferably constituting the surface layer (a)may comprise either an aliphatic polyester resin or an aromaticpolyester resin.

More specifically, examples of dicarboxylic acids constituting thepolyester resin may include: terephthalic acid, isophthalic acid,phthalic acid, 5-t-butylisophthalic acid, naphthalenedicarboxylic acid,diphenyl ether dicarboxylic acid, cyclohexane-dicarboxylic acid, adipicacid, oxalic acid, malonic acid, succinic acid, agelaic acid, sebacicacid, and dimer acids comprising dimers of unsaturated fatty acids.These acids may be used singly or in combination of two or more species.Examples of diols constituting the polyester resin may include: ethyleneglycol, propylene glycol, tetramethylene glycol, neopentyl glycol,hexamethylene glycol, diethylene glycol, polyalkylene glycol,1,4-cyclohexane-dimethanol, 1,4-butanediol, and 2-alkyl-1,3-propanediol. These diols may be used singly or in combination of two or morespecies.

Among these, it is preferred to use an aromatic polyester resinincluding an aromatic dicarboxylic acid component, particularlypreferably a polyester formed from terephthalic acid as the dicarboxylicacid and a diol having at most 10 carbon atoms, such as polyethyleneterephthalate or polybutylene terephthalate. It is also preferred to usea co-polyester resin formed by replacing a portion, preferably at most30 mol. %, more preferably at most 15 mol. %, of the terephthalic acidwith another dicarboxylic acid, such as isophthalic acid, or acopolyester resin between terephthalic acid and a mixture of diols, suchas ethylene glycol and 1,4-cyclohexanediol (e.g., “Kodapack PET#9921”,available from Eastoman Kodak Co.).

The polyester resin may preferably be one having an intrinsic viscosityof ca. 0.6–1.2. The outer surface layer (a) can contain up to 20 wt. %of a thermoplastic resin other than the polyester resin, such as athermoplastic elastomer as represented by thermoplastic polyurethane, ora polyolefin resin modified with an acid, such as maleic acid, or ananhydride thereof.

The thickness of the surface layer (a) comprising a thermoplastic resinother than polyamide resin may preferably be smaller than that of theintermediate layer (a), particularly at least 6% and below 50% of thatof the intermediate layer (b), so as not to impair the excellentstretchability and mechanical properties of the intermediate layer (b)comprising a polyamide resin.

Examples of the polyamide resin (PA) constituting the intermediate layer(b) may include: aliphatic polyamides, such as nylon 6, nylon 66, nylon11, nylon 12, nylon 69, nylon 610 and nylon 612; and aliphaticco-polyamides, such as nylon 6/66, nylon 6/69, nylon 6/610, nylon66/610, and nylon 6/12. Among these, nylon 6/66 and nylon 6/12 areparticularly preferred in view of moldability and processability. Thesealiphatic (co-)polyamides may be used singly or in mixture of two ormore species. It is also possible to use a blend of such an aliphatic(co-)polyamide with a minor amount of an aromatic polyamide. Herein, thearomatic polyamide means a polycondensation product between a diamineand a dicarboxylic acid, at least one of which contains at leastpartially an aromatic unit. An aromatic co-polyamide is preferred.Examples thereof may include: a copolymer of an aliphatic nylon and anaromatic polyamide including an aromatic diamine unit, such as nylon66/610/MXD6 (wherein “MXD6” represents polymetaxylylene adipamide), anda copolymer of an aliphatic nylon and an aromatic polyamide including anaromatic carboxylic acid unit, such as nylon 66/69/6I, nylon 6/6I andnylon 6I/6T (wherein “(nylon) 6I” represents polyhexamethyleneisophthalamide, and “(nylon) 6T” represents polyhexamethyleneterephthalamide). These polyamide resins may be used singly or inmixture so as to provide a melting point of preferably 160–210° C. Theintermediate layer (b) can contain up to ca. 30 wt. % of a thermoplasticresin other than the polyamide resin, such as a polyolefin resinmodified with an acid, such as maleic acid, or an anhydride thereof,ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer,ionomer resin, or (partially) saponified ethylene-vinyl acetatecopolymer.

The sealable resin constituting the inner surface layer (c) may beappropriately selected from thermoplastic resins inclusive of:polyolefins polymerized by using a single-site catalyst or metallocenecatalyst (sometimes abbreviated as “SSC”) inclusive of linearlow-density polyethylene (abbreviated as “SSC-LLDPE”) and verylow-density polyethylene (abbreviated as “SSC-VLDPE”); conventionaltypes of ethylene-α-olefin copolymers inclusive of “LLDPE” and “VLDPE”in terms of generally accepted abbreviations; ethylene-vinyl acetatecopolymer (abbreviated as “EVA”), ethylene-methacrylic acid copolymer(abbreviated as “EMAA”), ethylene-methacrylic acid-unsaturated aliphaticcarboxylic acid copolymer, low-density polyethylene, ionomer resin(abbreviated as “IO (resin)”), ethylene-acrylic acid copolymer,ethylene-methyl acrylate copolymer (abbreviated as “EMA”), andethylene-butyl acrylate copolymer (abbreviated “EBA”). Such a preferredclass of sealable resins may be termed as an ethylene copolymer,typically a copolymer of a major amount (i.e., more than 50 wt. %) ofethylene with a minor amount (i.e., less than 50 wt. %, preferably up to30 wt. %) of a vinyl monomer copolymerizable with ethylene selected fromthe group consisting of α-olefins having 3 to 8 carbon atoms, andunsaturated carboxylic acids and unsaturated esters of carboxylic acidshaving up to 8 carbon atoms, inclusive of acrylic acid, methacrylicacid, acrylate esters, methacrylate esters and vinyl acetate, or anacid-modified product of the ethylene copolymer (preferably modifiedwith up to 3 wt. % of an unsaturated carboxylic acid). It is alsopossible to use a thermoplastic resin, such as thermoplastic resin, suchas polypropylene resin, polyester resin or aliphatic nylon. The sealableresin may preferably have a melting point of at most 150° C., morepreferably at most 135° C. It is also possible to use a blend includingat least one species of such a sealable resin within an extent of notimpairing the transparency of the resultant film or a sealed productthereof.

Among the above, preferred examples of such sealable resins constitutingthe inner surface layer (c) may include: SSC-LLDPE, SSC-VLDPE, LLDPE,VLDPE, EVA, EMAA, ethylene-methacrylic acid-unsaturated aliphaticcarboxylic acid copolymer, and IO resins. A particularly preferred classof SSC-type polyolefins may include those obtained by using aconstrained geometry catalyst (a type of metallocene catalyst developedby Dow Chemical Company). The constrained geometry catalyst may provideethylene-α-olefin copolymers which may be classified as a substantiallylinear polyethylene resin having ca. 0.01–ca. 3, preferably ca. 0.01–ca.1, more preferably ca. 0.05–ca. 1, long-chain branching(s) per 1000carbon atoms. Because of long-chain branches each having ca. 6 or morecarbon atoms selectively introduced into its molecular structure, theethylene-α-olefin copolymer may be provided with excellent physicalproperties and good formability or processability, and an examplethereof is commercially available from Dow Chemical Company under atrade name of “AFFINITY” or “ELITE” (including 1-octene as α-olefin).

Other examples of polyethylene resins obtained by using a metallocenecatalyst may include those available under trade names of “EXACT” (EXXONCo.), “UMERIT” (Ube Kosan K.K.), “EVOLUE” (Mitsui Kagaku K.K.),“COLONEL” (Nippon Polychem K.K.) and “HARMOLEX” (Nippon PolyolefinK.K.).

Such a metallocene-catalyzed polyolefin (SSC-polyolefin) may preferablyhave a polydispersity index defined as a ratio (Mw/Mn) between aweight-average molecular weight (Mw) and a number-average molecularweight (Mn) of below 3, more preferably 1.9–2.2.

The surface layer (c) comprising a sealable resin may preferably have aheat resistance which is lower than that of the surface layer (a). Thisis because, if the surface layer (c) comprising a sealable resin has ahigher heat resistance than the surface layer (a), at the time ofapplying heat to the film for sealing or deep drawing, the surface layer(a) is liable to be melted in contact with a heating plate to result ina problem regarding the adaptability to sealing or packaging machine ordeep drawing processability.

The surface layer (c) comprising a sealable resin can be provided witheasy peelability, e.g., in the case of deep drawing packaging. This canbe accomplished by using, e.g., a mixture of EMAA and a polypropyleneresin, or a mixture of EVA and polypropylene resin.

The heat-shrinkable multilayer film according to the present inventionincludes the above-mentioned surface layer (a) comprising athermoplastic resin, intermediate layer (b) comprising a polyamideresin, and surface layer (c) comprising a sealable resin, as itsindispensable component layers, but can also include an additionalintermediate layer other than the intermediate layer (b) comprising apolyamide resin for the purpose of, e.g., providing the productmultilayer film with improved functionality or processability. Examplesof such an optional intermediate layer may include the following.

A gas barrier intermediate layer (d), particularly an-oxygen gas-barrierlayer, comprising a gas barrier resin, examples of which may include:EVOH; aromatic polyamides including an aromatic diamine unit, such aspolymethacrylene adipamide (nylon MXD6); and amorphous aromaticpolyamides including an aromatic carboxylic acid unit, such aspolyhexamethylene isophthalamide/terephthalamide (nylon 6I/6T) which isa copolymer of isophthalic acid, terephthalic acid andhexamethylenediamine.

Another type of preferable intermediate layer may comprise a copolymerof ethylene and at least one species of monomer containing an oxygenatom in its molecule. Specific examples thereof may include: EVA, EMAA,ethylene-methacrylic acid-unsaturated aliphatic carboxylic acidcopolymer, EMA, EAA, EBA and IO resin.

Further, a layer of metallocene-catalyzed polyolefin having a densitybelow 0.900 g/cm₃ exhibits a good stretch orientation characteristic andmay preferably be inserted as an optional intermediate layer forproviding a multilayer film having a large heat-shrinkability at a stageafter the biaxial stretching.

One or more adhesive resin layers may be inserted as an optionalintermediate layer, as desired, e.g., in case where a sufficientadhesion is not ensured between the above-mentioned respective layers.Such an adhesive resin can be selected from those constituting theabove-mentioned optional intermediate layers. Further preferred examplesof the adhesive resin used for the above purpose may include: EVA, EEA,EAA, acid-modified polyolefins (inclusive of reaction products betweenolefin homo-or co-polymers and unsaturated carboxylic acids, such asmaleic acid and fumaric acid, acid anhydrides, esters or metal salts ofthese acids, such as acid-modified VLDPE, acid modified LLDPE andacid-modified EVA). It is particularly suitable to use a polyolefinresin modified with an acid such as maleic acid or an anhydride thereof.

Into any one or more of the above-mentioned layers, it is possible toadd an additive, such as a lubricant or an antistatic agent.

Examples of the lubricant may include: hydrocarbon lubricants, fattyacid lubricants, fatty acid amide lubricants, ester lubricants andmetallic soaps. The lubricants may be liquid or solid. Specific examplesof the hydrocarbon lubricants may include: liquid paraffin, naturalparaffin, polyethylene wax and micro-crystalline wax. Fatty acidlubricants may include stearic acid and lauric acid. Fatty acid amidelubricants may include: stearic acid amide, palmitic acid amide,N-oleyl-palmitic acid amide, behenic acid amide, erucic acid amide,arachidic acid amide, oleic acid amide, methylene-bis-stearoyl amide,and ethylene-bis-stearoyl amide. Ester lubricants may include butylstearate, hardened castor oil, ethylene glycol monostearate, and stearicacid mono-glyceride. Metallic soaps may be derived from fatty acidshaving 12–30 carbon atoms and may include zinc stearate and calciumstearate as representative examples. Among these, fatty acid amidelubricants and metallic soaps may be preferred because of goodcompatibility with a thermoplastic resin, particularly a polyolefinicresin. Specifically preferred examples of lubricants may include behenicacid amide, oleic acid amide and erucic acid amide. These lubricants maypreferably be added in the form of a master batch. Such a master batchcontaining, e.g., 5–20 wt. % of a lubricant, may preferably be added inan amount sufficient to provide a concentration of 0.05–2 wt. % of thelubricant in a resin layer concerned.

The antistatic agent may preferably be a surfactant, which may be any ofanionic surfactants, cationic surfactants, nonionic surfactants,amphoteric surfactants and mixtures of these. The anti-static agent maypreferably be added in a proportion of 0.05–2 wt. %, more preferably0.1–1 wt. % of a resin layer to which it is added.

Preferred examples of layer structure of the stretch-oriented multilayerfilm according to the present invention are shown below. These arehowever not exhaustive.

-   (1) polyester resin/adhesive resin/polyamide resin/adhesive    resin/sealable resin,-   (2) polyester resin/adhesive resin/polyamide resin/gas barrier    resin/adhesive resin/sealable resin,-   (3) polyester resin/adhesive resin/polyamide resin/adhesive    resin/gas barrier resin/adhesive resin/sealable resin,-   (4) polyester resin/adhesive resin/polyamide resin/adhesive    resin/gas barrier resin/adhesive resin/polyamide resin/adhesive    resin/sealable resin,-   (5) polyester resin/adhesive resin/polyamide resin/gas barrier    resin/polyamide resin/adhesive resin/ sealable resin,-   (6) polyolefin resin/adhesive resin/polyamide resin/adhesive    resin/sealable resin,-   (7) polyolefin resin/adhesive resin/polyamide resin/gas barrier    resin/adhesive resin/sealable resin,-   (8) polyolefin resin/adhesive resin/polyamide resin/adhesive    resin/gas barrier resin/adhesive resin/ sealable resin,-   (9) polyolefin resin/adhesive resin/polyamide resin/adhesive    resin/gas barrier resin/adhesive resin/polyamide resin/adhesive    resin/sealable resin, and-   (10) polyolefin resin/adhesive resin/polyamide resin/gas barrier    resin/polyamide resin/adhesive resin/sealable resin.

The stretch-oriented multilayer film may preferably be formed bylaminating the above-mentioned layers, followed by stretching andrelaxation into a final form of multilayer film having a total thicknessof 20–250 μm, particularly 40–150 μm. Further, in the case of packagingrib (meat) retaining a sharp-cut bone and requiring an especially highpinhole resistance, the total thickness may preferably be 60–250 μm,particularly 90–150 μm.

More specifically, it is preferred that the surface layer (a) comprisinga thermoplastic resin has a thickness of 0.5–25 μm, particularly 1–15μm, the intermediate layer (b) comprising a polyamide resin has athickness of 3–50 μm, particularly 10–40 μm, and the surface layer (c)comprising a sealable resin has a thickness of 10–150 μm, particularly15–60 μm. Particularly, in the case where the surface layer (a)comprises a polyester resin, it is preferred that the layer (a) has athickness smaller than that of the layer (b), more specifically athickness of 3–70%, particularly 6–50%, of that of the layer (b), inorder to provide the multilayer film with a properly harmonized biaxialstretchability.

The optionally disposed gas barrier layer (d) may have a thickness of,e.g., 1–30 μm, preferably 2–15 μm. Below 1 μm, the oxygen gasbarrier-improving effect may be scarce, and above 30 μm, the extrusionof the layer and the stretching and processing of the multilayer filmbecome difficult.

The adhesive resin layer can be disposed in a plurality of layers, eachhaving a thickness in the range of suitably 0.5–5 μm.

The stretch-oriented multilayer film may be formed by first forming ayet-unstretched film by co-extrusion through a plurality of extrudersand then biaxially stretching the film by a known process, such as thetenter process, followed by a high degree of relaxation heat treatmentat a relaxation ratio of at least 20% in at least one axial direction.The stretching ratio may preferably be 2.5–4 times in both longitudinaland transverse directions. The thus-formed stretch-oriented multilayerfilm can also be laminated with another resin layer according to a knownlamination process.

The stretch-oriented multilayer film may preferably be formed throughinflation according to the process of the present invention. A preferredembodiment thereof is described with reference to FIG. 1, the solefigure in the drawing.

A number of extruders 1 (only one being shown) are providedcorresponding to the number of laminated resin species, and therespective resins from the extruders are co-extruded through an annulardie 2 to form a tubular product (parison) 3 including at least threelayers of an outer surface layer (a) comprising a thermoplastic resin,an intermediate layer (b) comprising a polyamide resin and an innersurface layer (c) comprising a sealable resin. The parison 3 is thenvertically pulled down into a water bath 4 and taken up by pinch rollers5 while being cooled down to a temperature that is below the lowest oneof the melting points of the principal resins constituting therespective resin layers (i.e., the thermoplastic resin, the polyamideresin and the sealable resin), preferably to 40° C. or below. Thethus-taken-up tubular film 3 a, while optionally introducing an openingaid such as soybean oil thereinto as desired, is introduced into a bath6 of warm water at, e.g., 80–95° C., which is at most the lowest one ofthe meting points of the principal resins constituting the respectivelayers, and the thus-warmed tubular film 3 b is pulled upwards to form abubble of tubular film 3C with fluid air introduced between pairs ofpinch rollers 7 and 8, whereby the tubular film 3C is biaxiallystretched simultaneously at a ratio of preferably 2.5–4 times, morepreferably 2.8–4 times, in each of vertical or machine direction (MD)and transverse or lateral direction (TD), most preferably at 2.9–3.5times (MD) and 3–3.5 times (TD), while cooling the film 3C with cool airat 10–20° C. from a cooling air ring 9. The thus biaxially stretchedfilm 3 d is once folded or laid flat and then pulled downwards to againform a bubble of tubular film 3 e with fluid air introduced betweenpairs of pinch rollers 10 and 11. The bubble of tubular film 3 e is heldwithin a heat-treating tube 12 wherein steam from blowing ports 13 isblown (or warm water from spraying ports is sprayed) against the tubularfilm 3 e to heat-treat the tubular film 3 e after the biaxial stretchingat 70–98° C., preferably 75–95° C., for ca. 1–20 sec., preferably ca.1.5–10 sec., thereby allowing the tubular film to relax by 15–40% (butat least 20% in at least one direction), preferably 20–35%, in each ofthe machine direction (MD) and the transverse direction (TD). A tubularfilm 3 f after the heat-treatment corresponds to a stretch-orientedmultilayer film according to the present invention and is wound about atake-up or winding roller 14.

Again to say, in order to realize improvements in various propertiesrepresented by an improved low-temperature impact resistance whileretaining excellent strengths, it is extremely preferred to adopt acombination of high degree of stretching and high degree of relaxationtreatment, i.e., to ensure high stretching ratios of 2.5–4 times, morepreferably 2.8–3.5 times, in both MD/TD, most preferably 2.9–3.5 times,in MD and 3–3.5 times in TD and then to effect a heat-treatment forcausing relaxation by 15–40% in each of MD/TD (but at least 20% in atleast one direction), preferably by 20–30% in each of MD/TD, with steamor warm water as a heating medium having a large heat capacity. At alower stretching ratio, it is difficult to attain necessary filmstrengths after the heat treatment, and the resultant film is liable tohave thickness irregularity, thus failing to exhibit satisfactorypackaging performance. On the other hand, in the case of using a heatingmedium having a small heat capacity, such as hot air, or adopting alower heat treatment temperature of below 70° C., it becomes difficultto realize a sufficiently large degree of relaxation, thus being liableto fail in realizing a necessary improvement in low-temperature impactresistance. On the contrary, if the heat treatment is effected at ahigher temperature exceeding 100° C., the sealable resin layer (c)comprising, e.g., a polyolefin, is liable to be melted, whereby theorientation of the layer (c) is removed, thus being liable to fail inproviding excellent strength. If the relaxation percentage is below 15%at the time of the heat treatment, it is difficult to realize asufficient degree of orientation relaxation at the amorphous portion asrepresented by a desired low-temperature impact resistance. Above 40%,the resultant film is liable to be wrinkled.

The thus-obtained stretch-oriented multilayer film according to thepresent invention retains a high degree of basic strength represented bya tensile strength as a result of the high degree of stretching of thepolyamide resin layer and is also provided with a remarkably improvedlow-temperature impact resistance represented by an impact energy at−10° C. Further, accompanying the increase in low-temperature impactresistance, the film has been also provided with remarkable improvementsin piercing strength, anti-pinhole property, deep drawingcharacteristic, etc. Through the high degree of relaxation heattreatment, the heat-shrinkability of the product multilayer film isnaturally lowered. Thus, the stretch-oriented multilayer film of thepresent invention does not include a heat-shrinkability as an essentialproperty but may preferably retain a certain degree of hot-watershrinkability depending on the use thereof since such a degree of hotwater shrinkability provides an improved appearance by preventing theoccurrences of winkles of a packaged product.

Examples of appropriate degrees of hot-water shrinkability (at 90° C.)for specific packaging materials may include: 0–20%, more preferably0–15%, for freeze packaging material; 0–25%, more preferably 0–15%, fordeep drawing packaging material; 5–20%, more preferably 5–15%, for traypackaging lid material; and below 15%, more preferably below 10% (below15%, more preferably below 10% in terms of dry heat-shrinkability at120° C.) for vertical pillow packaging material. Such a level ofhot-water shrinkability of a product stretch-oriented multilayer filmcan be controlled within an extent of retaining necessarylow-temperature impact resistance by adjusting the relaxation percentage(within an extent of ensuring at least 20% in at leas one direction) inconnection with the preceding stretching ratio.

In order to provide a freeze-packaged product or a packaged product forcold circulation around 5° C. (or 0–10° C.) with an improved pinholeresistance, the stretch-oriented multilayer film of the presentinvention may preferably show an actual impact resistance (i.e., not anormalized impact resistance at a thickness of 50 μm) of at least 1.6Joule at −10° C.

Particularly in the case of packaging of rib (meat) retaining asharp-cut bone and requiring an especially high pinhole resistance, themultilayer film may preferably show an actual impact resistance (asmeasured at −10° C.) of at least 3 Joule, more preferably at least 4Joule, further preferably at least 5 Joule, so as to allow the packagingwithout using a bone guard (reinforcing material) ordinarily used forsuch packaging.

In the above-described stretch-oriented multilayer film productionprocess according to the present invention, the multilayer film beforeor after the stretching may be exposed to radiation. By the exposure toradiation, the product multilayer film may be provided with improvedheat resistance and mechanical strength. Because of a moderatecrosslinking effect thereof, the exposure to radiation can exhibit aneffect of providing improved film formability by stretching and improvedheat resistance. In the present invention, known radiation, such as αrays, β rays, electron beams, γ rays, or X rays may be used. In order toprovide an adequate level of crosslinking effect, electron rays and γrays are preferred, and electron beams are particularly preferred inview of facility of handling and high processing capacity in producingthe objective multilayer film.

The conditions for the above exposure to radiation may be appropriatelyset depending on the purpose thereof, such as a required level ofcrosslinkage. For example, it is preferred to effect the electron beamexposure at an acceleration voltage in the range of 150–500 kilo-voltsto provide an absorbed dose of 10–200 kGy (kilo-gray) or effect γ-rayexposure at a dose rate of 0.05–3 kGy/hour to provide an absorbed doseof 10–200 kGy.

It is also possible that the inner surface or/and the outer surface ofthe stretch-oriented multilayer film of the present invention aresubjected to corona discharge treatment, plasma treatment or flametreatment.

The stretch-oriented multilayer film of the present invention has aremarkably improved low-temperature impact resistance and isparticularly suitable for use as a freeze packaging material. However,the presumably-caused extreme orientation relaxation at the amorphousportion represented by the improved low-temperature impact resistance incombination with the high degree of orientation of the crystallineportion has resulted in softness and improvements in piecing strength,etc., which have not been achieved heretofore. As a result, thestretch-oriented multilayer film of the present invention is alsoextremely suitable for use as, e.g., deep drawing packaging material,vertical pillow packaging material, tray packaging lid material, andpackaging material for cold or refrigeration circulation or freezepackaging material for rib (meat), fish meat and marine products such ascrabs, for which the above-mentioned properties are particularlydesired.

EXAMPLES

Hereinbelow, the present invention will be described more specificallybased on Examples and Comparative Examples. It should be noted howeverthat the scope of the present invention is not restricted by suchExamples. Some physical properties described herein are based on valuesmeasured according to the following methods.

<Physical Property Measurement Methods>

1. Impact Strength and Energy

Measured at −10° C. according to ASTM D3763-86 by using “DROP-WEIGHTTESTER RTD-5000” (available from Rheometrics, Inc.)

More specifically, in an environment of −10° C., a sample ofstretch-oriented multilayer film cut into a square of 10 cm×10 cm isdisposed horizontally and sandwiched between a pair of clamps eachhaving a 3.8 cm-dia. circular opening with its surface layer (a)directed upwards. Onto the sample film at the opening, a plunger of 4 kgin weight and having a hemispherical tip portion of 1.27 cm in diameteris dropped at a speed of 333.33 cm/sec to measure a load applied to thedropping plunger and a displacement by a sensor, from which adisplacement-load curve is obtained. Based on the curve, a maximum loaduntil the breakage is read as an impact strength (F_(IP) (N), and anenergy absorbed by the film until the breakage is calculated to obtainan impact energy (E_(IP) (J)). Five sample films from each product filmare subjected to the above measurement, and the average values are takenas measured values.

Based on the above-measured impact energy (E_(IP) (J)) for a samplehaving a thickness t (μm), an impact energy normalized at a thickness of50 μm (E_(IP50) (J)) is calculated according to the following equation:E _(IP50)(J)=E _(IP)(J)×(50/t).2. Piercing Strength

In an environment of 23° C. and 50% RH, a piercing pin having ahemispherical tip having a radius of curvature of 0.5 mm attached to atensile tester (“TENSILON RTM-100”, available from Orientec K.K.) iscaused to pierce a sample multilayer film from its surface layer (a)side at a speed of 50 mm/min, thereby measuring a maximum value of forceapplied to the film until the breakage thereof as a piercing strength(F_(p) (N)).

3. Hot-water Shrinkability

A sample film on which marks are indicated at a distance therebetween of10 cm in each of a machine direction (MD) and a transverse direction(TD) perpendicular to the machine direction, is dipped for 10 sec. inhot water adjusted at 90° C. and then taken out therefrom, followed byimmediate quenching within water at room temperature. Thereafter, thedistance between the marks is measured and a decrease in distance isindicated in percentage of the original distance 10 cm. Five samplefilms from each product film are subjected to the above measurement, andthe average value of percentage decrease is indicated in each of the MDand TD.

4. Dry heat-Shrinkability

A 3 mm-thick corrugated board is placed on a rack, and a Geer oven(“Model MOG-600”, available from K. K. Robert) is placed thereon andheated to a prescribed temperature. Into the oven, a sample film onwhich marks are indicated at a distance therebetween of 10 cm in each ofMD and TD is placed. In this instance, the door of the oven isimmediately closed after the placement of the sample film so that thedoor opening period is restricted to be within 3 minutes. After the doorclosure, the sample film is left standing for 30 sec in the Geer ovenand then taken out for natural cooling. Thereafter, the distance betweenthe marks on the sample film is measured, and a decrease in distance isindicated in percentage of the original distance 10 cm. Five samplefilms from each product film are subjected to the above measurement, andthe average value of percentage decrease is indicated in each of the MDand TD.

Film Production Examples

Next, Examples and Comparative Examples for production ofstretch-oriented multilayer films are described. Resins used in thefollowing productions examples are inclusively shown in Table 1 togetherwith their abbreviations.

Example 1

By using an apparatus having an arrangement as roughly shown in FIG. 1,a tubular laminate product (parison) having a laminar structure from theouter to the inner layers of PET (3)/mod-VL (2)/NY-1(13)/EVOH (4)/mod-VL(2)/LLDPE (31) with thickness ratios of respective layers indicated inthe parentheses was co-extruded by extruding the respective resinsthrough a plurality of extruders 1 (only one being shown) respectivelyand introducing the melted resins to an annular die 2 to melt-bond therespective layers in the above-described order. The molten parison 3extruded out of the die 2 was quenched to 10–18° C. by a water bath 4 toform a flat tubular product 3 a. Then, the flat tubular product 3 a waspassed through a warm water bath 6 at 92° C. and formed into abubble-shaped tubular film 3 c, which was then biaxially stretched atratios of 3.4 times in MD and 3.4 times in TD by the inflation processwhile being cooled with cooling air at 15–20° C. from an air ring 9.Then, the biaxially stretched film 3 d was guided into a 2 meter-longheat-treating tube 12 to form a bubble-shaped tubular film 3 e, whichwas then heat-treated for 2 sec. with steam at 90° C. blown out of steamblowing ports 13, while being allowed to relax by 20% in MD directionand by 20% in TD direction, thereby providing a biaxially stretched film(stretch-oriented multilayer film) 3 f. The thus-obtained multilayerfilm exhibited a lay-flat width of 490 mm and a thickness of 55 μm.

The laminate structure, film production (stretching-relaxation)conditions, physical properties and packaging performances of thethus-obtained multilayer film are inclusively shown in Tables 2 to 5together with those of multilayer films obtained in other Examples andComparative Examples.

Examples 2–15 and Comparative Examples 1, 2, 4, 5 and 7

Various multilayer films were prepared in similar manners as in Example1 except that the laminar structures and film production(stretching-relaxation) conditions were respectively changed as shown inTables 2 to 4.

Comparative Example 3

A commercially available 15 μm-thick stretched film of nylon 6 (O-Ny 6)and a commercially available 60 μm-thick unstretched film ofethylene-vinyl acetate copolymer (EVA) was applied to each other to forma composite film.

Comparative Example 6

Respective resins were melt-extruded from a plurality of extruders andthe melt-extruded resins were introduced into a T-die to be melt-bondedso as to provide a laminar structure and thickness ratios as shown inTable 4, thereby forming a co-extruded unstretched film.

Each of the multilayer films obtained in the above Examples andComparative Examples was subjected to the above-mentioned measurement ofphysical properties and performance evaluation tests describedhereinafter. The results are inclusively shown in Tables 2 to 5described hereinafter.

<Performance Evaluation Tests>

1. Hot Fill Performance

A 200 mm-wide and 400 mm-long pouch was formed from a sample film, and ahot water of ca. 70° C. was poured thereinto to evaluate the hot fillperformance according to the following standard:

A: A shrinkage after the pouring of hot water was at most 5%, thusshowing adaptability to hot filling.

C: A shrinkage after the pouring of hot water exceeded 5%, thus showingnon-adaptability to hot filling.

2. Anti-pinhole Property

Each product film of Examples and Comparative Examples were formed intobags of 220 mm-width and 450 mm-length (inner sizes). Into each bag, afrozen tuna cut piece (with skin) of 800 g cooled to −50° C. wasvacuum-packaged in an environment of ca. 15° C. to obtain a packagedproduct. Twenty packaged product samples were prepared from each productfilm and packed into two foam styrol boxes (size: 390 mm-L×330 mm-W×260mm-H) together with dry ice so that each box contained 10 packagedproduct samples together with dry ice. The boxes were transported on antruck from Shizuoka prefecture to Ibaraki prefecture (over a distance ofca. 300 kilometers). Then, the packaged product samples were checkedwith respect the presence or absence of pinholes, and the percentage ofbroken bag was calculated by [(number of bags with pinholes)/20]×100,whereby each product film was evaluated based on the broken bagpercentage according to the following standard.

A: No bags with pinholes (Broken bag percentage=0%)

B: Broken bag percentage ≦5%

C: Broken bag percentage exceeded 5%, thus showing a problem inpractical utility.

The occurrence of pinholes showed a clear correlation with thelow-temperature impact resistance and the piercing strength, and theproduct films of Examples showed remarkably better anti-pinhole propertythan the laminate film including a substantially identical thickness ofnylon layer

(Comparative Example 3).

3. Vertical pillow packaging performance

Each sample film was evaluated with respect to a vertical pillowpackaging performance by using a vertical pillow packaging machine(“ONPACK 207 SG”, made by Orihiro K. K.) used for packing a liquid orpowdery product by intermittent packaging operation. For the test, 1 kgof water at room temperature was packed under the conditions of alongitudinal seal temperature of 160° C., a transverse seal temperatureof 170° C. and a film cut length of 280 mm.

The packaging performance was evaluated with respect to coming-off ofthe film and sticky adhesion onto a seal bar according to the followingstandard.

(Film Coming-off)

A: Continuous packaging was possible

C: The film came off the seal bar due to shrinkage, thus failing toachieve continuous packaging.

(Sticky Adhesion onto a Seal Bar)

A: Continuous packaging was possible without causing sticky adhesion ofthe film onto the seal bar.

C: Continuous packaging was impossible due to sticky adhesion of thefilm during the packaging operation.

4. Deep Drawing Performance (Base Sheet)

A deep drawing packaging test was performed by using each sample film asa base sheet together with the film of Example 3 as a lid material bymeans of a deep drawing packaging machine (“FV-603”, made by OhmoriKikai Kogyo K. K.), wherein each sample film as a base sheet was deeplydrawn at 100° C. by using a disk-shaped mold of 98 mm in diameter and 30mm in depth (except that a mold of 60 mm in depth was used in Example14). The evaluation was performed with respect to the following items.

(1) Formability

A: Formable without breakage

C: Deep drawing was impossible due to breakage

(2) Hot-water Shrinkability and Piercing Strength at the Drawn Corner.

A hot-water shrinkability and a piercing strength (N) were measured withrespect to the drawn corner of a formed base sheet sample.

(3) Wrinkles after Boiling

A packaged product of indefinitely shaped roasted pig meat was dipped inhot water at 90° C. for 10 sec., and then the presence or absence ofwrinkles on the package surface was checked.

A: The packaged product surface was free from wrinkles and exhibited abeautiful appearance.

C: The packaged product surface showed wrinkles, thus lowering thecommercial value.

(4) Abuse Test

Crylichical rubber sheets (weight=ca. 60 g/sheet) each having athickness of 5 mm and a diameter of 98 mm were packed to providepackaged products each containing 5 rubber sheets. The packaged productsamples were placed in a hexagonal tube box (which was supportedrotatably about a shaft extending horizontally to pierce the centers oftwo mutually parallel hexagonal sides of the box) and subjected to a6-angle rotation test (abuse test) for 10 min. in an environment of 5°C. The packaged product samples after the test were subjected tomeasurement of pinhole percentage.

(5) Rib (Meat) Packaging Test

Beef rib retaining a sharp-cut back bone was vacuum-packaged with samplefilm bags to form 20 package product samples, and the packaged productsamples were dipped for 1 sec. in hot water at 90° C. to be shrinked.Thereafter, the packaged products were subjected to a circulation testat 5° C. and then checked with respect to the occurrence of pinholes.The evaluation was performed according to the following standard.

A: The pinhole percentage was 5% or below.

C: The pinhole percentage substantially exceeded 5%.

TABLE 1 Component Resins Crystal melting Abbreviation Resin Maker (Tradename) point (° C.) Remarks ** Ny-1 nylon 6–66 copolymer MitsubishiEngineering Plastic 195 η_(rel) = 4.5  (wt. ratio = 80:20) K.K. (NOVAMID2430A1) Ny-2 nylon 6–69 copolymer EMS Co. 134 (GRILON BM13SBG) Ny-3nylon 6I–6T copolymer EMS Co. — (GRIVORY G21) PET ethyleneterephthalate-isophthalate Kanebo K.K. 228 η_(int) = 0.80 copolymer *1(BELPET IFG-8L) EVOH saponified ethylene-vinyl acetate copolymer KurarayK.K. 160 MFR = 6.5 g/10 min. (ethylene content = 48 mol %) (EVALEPG156B) VLDPE ethylene-hexene copolymer Sumitomo Kagaku K.K. 119 MFR =3.0 g/10 min. (d = 0.908 g/cm³) (SMIKASEN C53009) LLDPE ethylene-octenecopolymer Idemitsu Sekiyu Kagaku K.K. 122 MFR = 2.0 g/10 min. (d = 0.916g/cm³) (MORETEC 0238CN) SVL ethylene-octene copolymer *2 Dow ChemicalCo. 100 MFR = 1.0 g/10 min. (d = 0.902 g/cm³) (AFFINITY PL1880) mod-VLmodified very low density polyethylene *3 Mitsui Kagaku K.K. — MFR = 2.7g/10 min. (ADMER SF730) *1: Acid is a mixture of 12 mol % isophthalicacid and 88 mol % terephthalic acid. *2: Polymerized in the presence ofa metallocene catalyst. *3: Modified with an unsaturated carboxyliceacid. **: η_(rel) = relative viscosity, η_(int) = intrinsic viscosity,MFR = inlet flow rate.

TABLE 2 Freeze-packaging & Vertical pillow packaging (1) Ex. & Comp. Ex.Ex. 1 Ex. 2 Ex. 3 Ex. 4 Film structure 1st thickness (μm) PET 3 PET 2PET 2 PET 3 2nd mod-VL 2 mod-VL 2 mod-VL 2 mod-VL 1.5 3rd Ny-1 13 Ny-112 Ny-1 14 Ny-1 12 4th EVOH 4 EVOH 4 EVOR 9 EVOH 4 5th mod-VL 2 mod-VL 2mod-VL 2 mod-VL 1.5 6th LLDPE 31 LLDPE 32 LLDPE 35 LLDPE 18 total (μm)55 54 64 40 Stretch ratio MD 3.4 3 3 3 TD 3.4 3 3 3.2 Relaxation heatingTemp. (° C.) 90 90 90 90 Percentage (%) MD 20 25 15 20 TD 20 25 27 20(Film properties) Piercing strength (N) 28 23 27 23 Impact strength (N)307 214 291 241 Impact energy (J) measured 2.7 2.0 2.2 2.2 at 50 μm 2.51.8 1.7 2.8 Hot water shrink (%) MD 11 12 10 8 90° C. TD 15 8 8 12 Dryshrink (%) MD 10 7 6 5 120° C. TD 14 5 4 6 (Freeze packaging) Hot fill AA A A Anti-pinhole A B A A Broken bag percentage 0% 5% 0% — (Pillowpackaging) Film coming-off — — — — Sticking to seal bar — — — — Ex. &Comp. Ex. Ex. 5 Ex. 6 Ex. 7 Ex. 8 Film structure 1st thickness (μm) PET2 LLDPE 5 PET 3 PET 3 2nd mod-VL 2 mod-VL 1.5 mod-VL 2 mod-VL 2 3rd Ny-112 Ny-1 14 Ny-1 14 Ny-1 16 4th EVOH 4 EVOH 4 EVOH 4 EVOH 4 5th mod-VL 2mod-VL 1.5 mod-VL 2 mod-VL 2 6th LLDPE 30 SVL 20 LLDPE 34 VLDPE 40 total(μm) 52 46 59 67 Stretch ratio MD 3.2 3.2 3.2 3.2 TD 3.2 3.2 3.2 3.2Relaxation heating Temp. (° C.) 90 90 95 90 Percentage (%) MD 20 20 2825 TD 20 20 35 30 (Film properties) Piercing strength (N) 22 18 18 19Impact strength (N) 257 193 214 228 Impact energy (J) measured 2.3 1.81.9 2.3 at 50 μm 2.2 2.0 1.6 1.7 Hot water shrink (%) MD 8 6 2 6 90° C.TD 12 10 3 9 Dry shrink (%) MD 5 6 4 3 120° C. TD 7 8 4 4 (Freezepackaging) Hot fill — A A A Anti-pinhole — B A A Broken bag percentage —5% 0% 0% (Pillow packaging) Film coming-off A — A A Sticking to seal barA — A A

TABLE 3 Freeze-packaging & Vertical pillow packaging (2) Ex. & Comp. Ex.Ex. 9 Ex. 10 Comp. Ex. 1 Comp. Ex. 2 Comp. Ex. 3 Comp. Ex. 4 Comp. Ex. 5Film structure 1st thickness PET 2 PET 2 PET 1.5 LLDPE 3 O-Ny6 15 PET 4Ny-1 10 (μm) 2nd mod-VL 2 mod-VL 2 mod-VL 1.5 mod-VL 1.5 EVA 60 mod-VL1.5 mod-VL 2 3rd Ny-1 14 Ny-1 11 Ny-1 8 Ny-1 12 Ny-1 20 EVOH 3 4th EVOH4 EVOH 4 EVON 5 EVON 4 EVOH 3 TPU 3 5th mod-VL 2 mod-VL 2 mod-VL 1.5mod-VL 1.5 mod-VL 1.5 mod-VL 2 6th LLDPE 44 LLDPE 40 LLDPE 21 LLDPE 20VLDPE 50 VLDPE 20 total (μm) 68 61 38.5 42 75 80 42 Stretch ratio 3.23.2 3.1 3 2.4 2.9 MD TD 3.2 3.2 3.2 3.2 2.8 2.8 Relaxation heating Temp.(° C.) 90 90 70 90 70 none Percentage (%) 25 25 10 15 10 none MD TD 2525 10 15 10 none (Film properties) Piercing 20 19 17 17 14 22 15strength (N) Impact 239 218 160 160 143 225 176 strength (N) Impactenergy 2.2 1.9 0.9 1.0 0.8 1.5 1.2 (J) measured at 50 μm 1.6 1.6 1.2 1.20.5 0.9 1.4 Hot water 9 7 28 10 0 17 34 shrink (%) MD 90° C. TD 13 13 308 0 24 30 Dry shrink (%) 4 4 21 6 0 — — MD 120° C. TD 7 7 23 6 0 — —(Freeze packaging) Hot fill — — C A A — — Anti-pinhole — — C C C — —Broken bag — — 30% 30% 60% — — percentage (Pillow packaging) Filmcoming- A A C A — — — off Sticking to seal A A A C — — — bar

TABLE 4 Deep drawing Ex. & Comp. Ex. Ex. 11 Ex. 12 Ex. 13 Comp. Ex. 1Comp. Ex. 6 Comp. Ex. 7 Ex. 14 Film structure 1st thickness PET 2 PET 2PET 2 PET 1.5 Ny-1 22 LLDPE 3 PET 3 (μm) 2nd mod-VL 2 mod-VL 2 mod-VL 2mod-VL 1.5 mod-VL 10 mod-VL 1.5 mod-VL 2 3rd Ny-1 13 Ny-1 15 Ny-1 + 12Ny-1 8 EVOH 14 Ny-1 12 Ny-1 16 Ny-2 = 50 + 50 wt % 4th EVOH 7 EVOH 9EVON 4 EVON 5 Ny-1 44 EVOH 4 EVOH 8 5th mod-VL 2 mod-VL 2 mod-VL 1.8mod-VL 1.5 mod-VL 10 mod-VL 1.5 mod-VL 2 6th LLDPE 30 LLDPE 35 LLDPE 30LLDPE 21 LLDPE 101 LLDPE 18 LLDPE 40 total (μm) 56 65 52 38.5 200 40 71Stretch ratio 2.9 3 3.6 3.1 none 3 3.2 MD TD 3 3 3.2 3.2 none 3.2 3.2Relaxation heating Temp. (° C.) 90 90 90 70 none 90 95 Percentage (%) 2015 20 10 none 15 30 MD TD 20 27 20 10 none 15 32 (Film properties)Piercing 21 27 20 17 17 20 strength (N) Impact 238 291 214 160 160 250strength (N) Impact energy 2.0 2.2 1.8 0.9 1.0 2.3 (J) measured at 50 μm1.8 1.7 1.7 1.2 1.3 1.6 Hot water 7 10 12 28 0 10 3 shrink (%) MD 90° C.TD 13 8 15 30 0 8 5 (Deep drawing (base sheet)) Formability A A A C A AA Surface gloss A A A A C A Maximum 24 24 27 bro- 29 50 drawn depth kenPiercing 1.8 2.1 1.6 1.5 — strength (N) Hot water 22 19 30 5 — shrinkWrinkles A A A C A Abuse test 0% 0% 0% 0% 0%

TABLE 5 Rib (meat) packaging Ex. & Comp. Ex. Ex. 15 Film structure 1stthickness (μm) PET 4 2nd mod − VL 2.5 3rd Ny − 1 + 34 NY − 3 = 80 + 20wt % 4th EVOH 5 5th mod − VL 2.5 6th VLDPE 80 total (μm) 128 Stretchratio MD 2.9 TD 3.3 Relaxation heating Temp. (° C.) 75 Percentage (%) MD25 TD 25 (Film properties) Piercing strength (N) 38 Impact strength (N)440 Impact energy (J) measured 5.9 at 50 μm 2.3 Hot water shrink (%) MD20 90° C. TD 24 Dry shrink (%) MD — 120° C. TD — (Rib packaging) Pinholeresistance A

INDUSTRIAL APPLICABILITY

As describe above, according to the present invention, it has becomepossible to produce a stretch-oriented multilayer film including apolyamide resin layer as a principal intermediate layer and having aremarkably improved low-temperature impact resistance while retainingnecessary strength through a combination of a high degree of stretchingand a high degree of relaxation heat treatment at degrees which have notbeen exercised heretofore. Accompanying the improvement inlow-temperature impact resistance, the stretch-oriented multilayer filmis provide with improvements in piercing strength, anti-pinholeproperty, etc., thus being suitably used not only as a freeze packagingmaterial but also as a deep drawing packaging material, a verticalpillow packaging material, and also a tray packaging lid material.

1. A stretch-oriented multilayer film, comprising at least three layers including a surface layer (a) comprising a thermoplastic resin, an intermediate layer (b) comprising a polyamide resin and a surface layer (c) comprising a sealable resin, said multilayer film exhibiting an impact energy of at least 1.5 Joule at a conversion thickness of 50 μm at −10° C.
 2. A multilayer film according to claim 1, exhibiting an actual impact energy at −10° C. of at least 1.6 Joule.
 3. A multilayer film according to claim 1, wherein the surface layer (a) has a larger heat resistance than the surface layer (c).
 4. A multilayer film according to claim 1, wherein the intermediate layer (b) has a larger thickness than the surface layer (a).
 5. A multilayer film according to claim 1, wherein the surface layer (a) comprises a stretch-oriented polyester resin.
 6. A multilayer film according to claim 1, having a heat-shrinkability.
 7. A multilayer film according to claim 6, having a hot-water shrinkability at 90° C. of below 20%.
 8. A multilayer film according to claim 6, having a hot-water shrinkability at 90° C. of below 15%.
 9. A freeze-packaging material, comprising a stretch-oriented multilayer film according to claim
 1. 10. A vertical pillow-packaging material, comprising a stretch-oriented multilayer film according to claim
 1. 11. A deep drawing-packaging material, comprising a stretch-oriented multilayer film according to claim
 1. 12. A tray-packaging lid material, comprising a stretch-oriented multilayer film according to claim
 1. 13. A process for producing a stretch-oriented multilayer film, comprising the steps of: co-extruding at least three species of melted thermoplastic resins to form a tubular product comprising at least three layers including an outer surface layer (a) comprising a thermoplastic resin other than polyamide resin, an intermediate layer (b) comprising a polyamide resin and an inner surface layer (c) comprising a sealable resin, cooling with water the tubular product to a temperature below a lowest one of the melting points of the thermoplastic resin, the polyamide resin and the sealable resin constituting the layers (a), (b) and (c), re-heating the tubular product to a temperature which is at most the lowest one of the melting points of the thermoplastic resin, the polyamide resin and the sealable resin constituting the layers (a), (b) and (c), vertically pulling the tubular product while introducing a fluid into the tubular product to stretch the tubular product in the vertical direction and the circumferential direction, thereby providing a biaxially stretched tubular film, folding the tubular film, again introducing a fluid into the folded tubular film to form a tubular film, heat-treating the tubular film from its outer surface layer (a) with steam or warm water until a relaxation ratio reaches at least 20% in at least one of the vertical direction and the circumferential direction, and cooling the heat-treated tubular film to provide a stretch-oriented multilayer film exhibiting an impact energy of at least 1.5 Joule at a conversion thickness of 50 μm at −10° C.
 14. A process according to claim 13 wherein the biaxially stretched tubular film is formed by stretching the tubular product at ratios of at least 2.9 times in a vertical direction and at least 3 times in a circumferential direction while vertically pulling the tubular product.
 15. A process according to claim 13, wherein the tubular film is heat-treated for the relaxation with steam or warm water at 75–95° C.
 16. A multilayer film according to claim 2, wherein the surface layer (a) has a larger heat resistance than the surface layer (c).
 17. A multilayer film according to claim 2, wherein the intermediate layer (b) has a larger thickness than the surface layer (a).
 18. A multilayer film according to claim 3, wherein the intermediate layer (b) has a larger thickness than the surface layer (a).
 19. A multilayer film according to claim 2, wherein the surface layer (a) comprises a stretch-oriented polyester resin.
 20. A multilayer film according to claim 3, wherein the surface layer (a) comprises a stretch-oriented polyester resin.
 21. A multilayer film according to claim 4, wherein the surface layer (a) comprises a stretch-oriented polyester resin.
 22. A multilayer film according to claim 2, having a heat-shrinkability.
 23. A multilayer film according to claim 3, having a heat-shrinkability.
 24. A multilayer film according to claim 4, having a heat-shrinkability.
 25. A multilayer film according to claim 5, having a heat-shrinkability.
 26. A freeze-packaging material, comprising a stretch-oriented multilayer film according to claim
 2. 27. A freeze-packaging material, comprising a stretch-oriented multilayer film according to claim
 3. 28. A freeze-packaging material, comprising a stretch-oriented multilayer film according to claim
 4. 29. A freeze-packaging material, comprising a stretch-oriented multilayer film according to claim
 5. 30. A freeze-packaging material, comprising a stretch-oriented multilayer film according to claim
 6. 31. A freeze-packaging material, comprising a stretch-oriented multilayer film according to claim
 7. 32. A freeze-packaging material, comprising a stretch-oriented multilayer film according to claim
 8. 33. A vertical pillow-packaging material, comprising a stretch-oriented multilayer film according to claim
 2. 34. A vertical pillow-packaging material, comprising a stretch-oriented multilayer film according to claim
 3. 35. A vertical pillow-packaging material, comprising a stretch-oriented multilayer film according to claim
 4. 36. A vertical pillow-packaging material, comprising a stretch-oriented multilayer film according to claim
 5. 37. A vertical pillow-packaging material, comprising a stretch-oriented multilayer film according to claim
 6. 38. A vertical pillow-packaging material, comprising a stretch-oriented multilayer film according to claim
 7. 39. A vertical pillow-packaging material, comprising a stretch-oriented multilayer film according to claim
 8. 40. A deep drawing-packaging material, comprising a stretch-oriented multilayer film according to claim
 2. 41. A deep drawing-packaging material, comprising a stretch-oriented multilayer film according to claim
 3. 42. A deep drawing-packaging material, comprising a stretch-oriented multilayer film according to claim
 4. 43. A deep drawing-packaging material, comprising a stretch-oriented multilayer film according to claim
 5. 44. A deep drawing-packaging material, comprising a stretch-oriented multilayer film according to claim
 6. 45. A deep drawing-packaging material, comprising a stretch-oriented multilayer film according to claim
 7. 46. A deep drawing-packaging material, comprising a stretch-oriented multilayer film according to claim
 8. 47. A tray-packaging lid material, comprising a stretch-oriented multilayer film according to claim
 2. 48. A tray-packaging lid material, comprising a stretch-oriented multilayer film according to claim
 3. 49. A tray-packaging lid material, comprising a stretch-oriented multilayer film according to claim
 4. 50. A tray-packaging lid material, comprising a stretch-oriented multilayer film according to claim
 5. 51. A tray-packaging lid material, comprising a stretch-oriented multilayer film according to claim
 6. 52. A tray-packaging lid material, comprising a stretch-oriented multilayer film according to claim
 7. 53. A tray-packaging lid material, comprising a stretch-oriented multilayer film according to claim
 8. 54. A process according to claim 14, wherein the tubular film is heat-treated for the relaxation with steam or warm water at 75–95° C. 